This invention relates to the processing of dewar assemblies, and, more particularly, to an approach that accomplishes the final cleaning, degassing, assembling, and joining of the dewar in an efficient manner.
One type of infrared sensor utilizes a focal plane array (FPA) of individual sensor elements. The sensor elements are typically photodiodes or photoconductors that produce an electrical output responsive to incident infrared energy. The sensor elements are arranged into an array of suitable size, which may comprise a few elements in a linear array or more than 1000 by 1000 elements in a two-dimensional array.
For the sensor elements to operate most effectively, they must be cooled to a reduced temperature of, for example, about 77K. Cooling of the sensor elements is accomplished by placing the focal plane array into an evacuated enclosure having an infrared-transmissive window. The enclosure, termed a vacuum package, is cooled to the required reduced temperature by cryogenic gas cooling, Joule-Thomson cooling, thermoelectric cooling, or other operable technique. The present invention relates to the method of the fabrication of the vacuum package and sensor system.
In the conventional practice, the vacuum package is formed of a two-part housing. A window housing includes the infrared-transmissive window, and a lower vacuum housing contains the focal plane array, a getter, and related structure. A tip-off tube extends from the side of the lower vacuum housing, and communicates with the interior of the housing.
To accomplish the assembly, the various pieces are carefully cleaned and vacuum baked to remove contaminants. The pieces are sub-assembled and again vacuum baked to remove contaminants. The getter is installed in the housing. The window housing is welded to the lower vacuum housing. A vacuum is drawn on the tip-off tube, and, after evacuation of the interior of the housing through the tip-off tube, the assembly is baked out to remove contaminants. The getter is activated by heating under vacuum. Finally, the tip-off tube is mechanically pinched together and closed off. The interior of the housing is thereby closed and isolated under vacuum. This approach is described in greater detail in U.S. Pat. No. 4,919,291.
The present inventors have recognized that the above-discussed approach, while operable, has disadvantages in some situations. Numerous vacuum bakeouts are required, with a corresponding long production time cycle. Each vacuum bakeout requires a long time to complete, because of the time required to evacuate through the tip-off tube. The repeated exposure to air between bakeouts allows re-adsorption of contaminants in the system. Supplemental cleaning is difficult. The stub of the tip-off tube that remains on the lower vacuum housing may pose a physical obstacle to mounting, external connecting, and other operations that are performed on the vacuum package assembly.
Thus, while the existing approach is operable, it could be improved so as to reduce production costs and to increase the reliability of the sensor system. The present invention provides such an improvement, and also provides related advantages.